For close to a century, microorganism content, e.g., bacteria and viruses, in municipal water supplies has been controlled through the addition of oxidative chemicals such as chlorine. This has proven effective in control of most microorganisms and is easily monitored. For example, a residual capable of being measured is carried throughout the municipal distribution system and periodically monitored to insure that the drinking water supply has been effectively treated. However, these systems are not always reliable or readily available to remote areas. Moreover, when an oxidizing agent is used at the source point, there can be contamination away from the source caused by pipeline problems that could allow the water to be unsafe at the time it arrives at the final point of use. In addition, there are also growing health concerns surrounding some of the compounds formed from the use of oxidative chemicals in the water supply.
To address contamination away from the source, a variety of devices or methods can be utilized to remove, destroy or deactivate microorganisms at the point of use. These include boiling the water, exposing the water to ultraviolet light, use of ozone, addition of chemicals and others. Most, if not all, of the methods used to remove, destroy and/or deactivate microorganisms include the need for external energy or the addition of chemicals to the water.
None of the known methods typically used to remove, destroy and/or deactivate microorganisms at the point of use can be used to remove sediment or turbidity from the source water. Typically, conventional filtration apparatuses are used in combination with a process or apparatus to destroy, deactivate and/or remove microorganisms. The filtration apparatuses are utilized primarily for removing particles in order to reduce turbidity. Examples of typical water filtration media include sand, garnet and anthracite.
In addition to bacteria and viruses present in the source, otter microorganisms that can be harmful include protozoan cysts. Removal of harmful cysts is desired and is reflected in the EPA filtration requirements now mandated by the Surface Water Treatment Rule. Since some of these cysts are not destroyed and/or deactivated effectively by the typical chlorine dosages used in municipal application, filtration and the use of chemical coagulants are typically used. The chemical coagulants increase the size of the particles containing the cysts to a point at which they can be removed by conventional filtration. During coagulation, small particles are agglomerated into larger particles by adding the chemical coagulants to the source. Once agglomerates of a desired size are produced, the solution is passed through a filter to filter out the agglomerates.
However, chemical coagulation has several disadvantages. The mechanism for filtering the liquid is by physically straining particles from the feed solution, which are larger than can pass through interstices between grains of the media. The media can only remove particles that are larger than the interstices. For example, sand filters can only remove particles greater than about 20 microns in size. Eventually, the particles held by the media seal off the interstices, reducing filtration efficiency. Moreover, chemical coagulation does not necessarily remove or deactivate all of the microorganisms present in the source water. Chemical coagulation is also disadvantageous in view of the cost of the chemicals, the need to regulate the amount of chemicals despite a continuously changing feed stream and in view of a low flow rate. Disposing of chemical sludge waste is another concern
Thus, there is a need for a method and apparatus that could simultaneously filter and disinfect a water supply without the need for external energy or addition of chemicals.